Project Summary Fragile X syndrome (FXS) is the most common inherited form of intellectual disability and a leading genetic cause of autism spectrum disorders (ASD). FXS is caused by the loss of functional fragile X mental retardation protein (FMRP). FMRP, encoded by the FMR1 gene, is a selective RNA-binding protein associated with translating polyribosomes. Previous works have focused on the role of FMRP as translational regulator and many mRNA targets of FMRP have been shown to be ASD-linked genes. Epigenetic regulation plays a pivotal role in neurodevelopment and neuropsychiatric disorders, including ASD. In addition to DNA and histone modifications, more than 150 post-transcriptionally modified ribonucleosides have been identified in various types of RNA. Among different RNA modifications, N6-methyladenosine (m6A) is by far the best-known modification on mRNA and lncRNA. m6A is dynamically regulated with dedicated writer, eraser, and reader proteins. m6A significantly affects RNA splicing, export, localization, translation efficiency and stability. We have found that the temporal and spatial distribution of m6A during neurodevelopment is highly dynamic, and the m6A-marked transcripts are enriched among ASD-linked genes as well as the mRNA targets of fragile X mental retardation protein (FMRP). Our biochemical analyses as well as the work of others have found that FMRP could bind to the m6A sites of its mRNA targets (m6A reader). Furthermore, we found that FMRP could maintain the stability of its mRNA targets, suggesting a new biological role of FMRP in gene regulation. Human-induced pluripotent stem cells (iPSCs) are pluripotent and are able to generate many different cell types. Three-dimensional (3D) aggregate culture of iPSCs has evolved from embryoid body culture, quite faithfully following human organogenesis, and provides a new platform to investigate human brain development in a dish, otherwise inaccessible to experimentation. Our preliminary data suggest that the loss of FMRP could alter the development of human forebrain organoids. Furthermore, recently published data suggest that m6A modification is more pervasive during neurodevelopment in human than mouse. In this proposed study, we will develop and characterize the human forebrain organoids of FXS and identify the potential human-specific mRNA targets of FMRP during human brain development. Our proposed works will lead to the development of the human forebrain organoid model for FXS and potentially identify the human- specific FMRP targets.